I get my feed from a farmer friend who grows and mixes it himself. part of it is ground, coarse grains, and part is powdery like you said. My hens do the same thing -- they pick out all the CORN bits without eating the rest and then look at me like "WE WANT MORE CORN". I've learned that corn is like candy to hens, and it is not a balanced diet for them to subsist on corn. So yes, in my case, and probably in yours, they need to also eat the powdery stuff which has the calcium, probiotics, protein, minerals etc. I don't put feed on the ground anymore, I only use those round feeders -- it wastes a lot less food, and they are kind of "forced" to eat the powder part to get more corn kernels. My turkeys on the other hand, gulp the powder down in huge gulps like it's going out of style. They are weird eaters though.